1. Field of the Invention
The present invention relates to a display device comprising a light emitting device such as an organic light emitting device, a display unit constructed from this display device, and an imaging device consisting of a combination of a display portion constructed from this display device and an imaging portion.
2. Description of the Related Art
As one of flat panel displays, an organic light emitting display using an organic light emitting device (organic EL (Electroluminescence) device) has been recently noted. The organic light emitting display has characteristics that its visual field angle is wide and its power consumption is low since it is a self-light emitting type display. The organic light emitting display is also thought of as a display having sufficient response to high-definition and high-speed video signals, and is under development toward the practical use.
As the organic light emitting device, for example, an organic light emitting device, wherein a first electrode, an organic layer including a light emitting layer, and a second electrode are sequentially layered over a substrate is known. In order to apply this device to a full color display, it is necessary to form fine pixel for emitting light of red, blue, and green, the three primary colors. As a method to form such pixel, method for individually coloring the pixel of the three primary colors by using a deposition mask, method for combining a white light emitting device and a color filter, method for combining a blue light emitting device and a fluorescence conversion layer (fluorescence conversion method) and the like can be cited.
In the white color filter method, there is a light loss when color separation is performed by the color filter. In general, the color filter cannot sharply cut a target wavelength, and has broad transmission characteristics. Therefore, for example, since wavelength bands of blue and green are close to each other, separation is difficult. In order to perfectly separate blue, a color filter having a high density is necessary, and transmittance of the color filter should be lowered.
In the fluorescence conversion method, conversion efficiency from the blue light emitting device to red is low. One of the reasons thereof is that while an excitation light source (EL light emission) is blue, an absorption wavelength band of a red conversion layer exists in the vicinity of orange. A method for resolving this problem by increasing overlap of excitation light and the absorption wavelength band is recently suggested. This method uses an EL spectrum, wherein a yellow light emitting component is added to the original blue light emission. Thereby, a red element from the yellow light emission is added to the red light emission from the fluorescence conversion layer with the improved conversion efficiency, and intensity of the red light emission is improved as a whole.
However, in this method, there is a problem that the yellow light emitting component added for the overlap becomes a cause to lower color purity when blue, red, and green are separated by the color filter. When the color purity is tried to be raised, it cannot be avoided that efficiency is further lowered. For example, in the case of red, a color filter for sufficiently cutting the yellow light emitting component is required to raise the color purity. Similarly, in the case of green, the yellow light emitting component close to green needs to be cut by a color filter having a sufficient density, leading to lowering transmittance of the color filter.
As above, in order to perform color separation into red, green, and blue light having good color purity by the color filter by using the EL spectrum having intensity also for the wavelength band slightly shifted from red, green, and blue, a color filter having a higher density is required. The reason thereof is that practical color filter characteristics are broad transmission characteristics centering on red, green, and blue. As a result, as long as the color separation is performed by the color filter, it is not avoidable that the transmittance of the color filter is lowered when a light emitting component other than desired red, green, and blue is included in the EL spectrum, leading to lowering total efficiency.
In this method, the conversion efficiency of red is still low. Therefore, a thick red fluorescence conversion layer is required, and therefore steps cannot be simplified.